The present invention relates to pneumatic brake boosters of the tandem type for the brake boosting of motor vehicles.
Pneumatic brake boosters of the tandem type are well known to an average person skilled in the art, since, in general terms, they make it possible to increase the brake boosting force, without appreciably increasing the outside diameter of the brake booster.
For example, the document U.S. Pat. No. 4,494,445 describes such a brake booster comprising a movable hub driving a push rod by means of a reaction disk, a closed containment composed of a front shell on the same side as the master cylinder and of a rear shell on the same side as the actuating pedal, both having a bottom and a cylindrical ring, defining a front chamber and a rear chamber separated by a separating element having an annular partition, and a front piston and a rear piston, respectively arranged coaxially in the front and rear chambers which are themselves divided respectively by a sealing diaphragm fixed to each of the pistons in order to define, within each of these, a sub-chamber of constant pressure and a sub-chamber of variable pressure, the pistons being movable in the axial direction under the effect of the differences in the pressures prevailing between the sub-chambers of each chamber.
Such a booster has a considerable disadvantage in that, when air is admitted into the sub-chamber of variable pressure of the front chamber, the difference between the pressure prevailing in this sub-chamber and the pressure prevailing in the sub-chamber of constant pressure of the rear chamber adjacent to it tends to tear off the separating element. The latter therefore has to be fastened firmly to the front shell, thereby necessarily complicating the production of such a booster, while at the same time increasing its cost and reducing its reliability.
Moreover, since the front piston is downstream of the reaction disk and is fastened to the push rod, it does not participate in the reaction which the driver must feel as a function of the boost.
Finally, the return spring is seated in one of the sub-chambers and is therefore of reduced length. Its rigidity consequently has to be very high. Because of the broad spread of characteristics of springs of high rigidity, it is difficult for boosters having identical characteristics to be obtained in series.
U.S. Pat. No. 4,279,193 makes known a pneumatic brake booster of the tandem type, in which the annular partition of the separating element is locked in a peripheral zone projecting from the part in the form of a cylindrical ring of one of the shells. Consequently, the brake booster has a large volume, and moreover, in the event of pneumatic failure, the gasket between the separating element and the rear end of the front piston determines a resistance which the driver has to overcome.